Designers of hybrid vehicle systems are able to shut down operation of an internal combustion engine during vehicle stops to improve fuel economy. When the vehicle employs a conventional hydraulic automatic transmission, an electrically-driven auxiliary hydraulic pressure pump can be implemented to provide hydraulic pressure to a forward clutch pack during periods when the internal combustion engine is shutdown. Maintaining transmission pressure ensures that during subsequent operation of the vehicle, the forward clutch is engaged, to provide a smooth launch similar to that found with a conventional vehicle.
Referring now to FIG. 1, a graphical representation of exemplary operation of a vehicle during an engine shutdown event is shown, including operation of a motor/generator unit (‘MGU’), vehicle speed (‘Veh Spd’) and voltage supplied to the transmission auxiliary pump (‘VT—AUX’), each shown as a function of time. During ongoing vehicle operation with the engine operating, voltage supplied to the transmission auxiliary pump or VT—AUX, is 0.0 V, as shown at the portion of the VT—AUX line labeled as A. When the internal combustion engine is shutdown, voltage to the electrically-driven auxiliary hydraulic pressure pump is boosted, e.g. to 12.0V, to boost hydraulic pressure and minimize pressure dips during a transition from a time at which hydraulic pressure to transmission clutches is provided by a mechanically-driven pump in the transmission until the hydraulic pressure is provided by the electrically-driven auxiliary hydraulic pressure pump. This is shown at the portion of the VT—AUX line labeled as B. During engine-off, auxiliary pump voltage, and hence hydraulic pressure, is maintained at a steady value (e.g. 9.0V) to balance clutch pressure, energy consumption, and pump durability, as shown at the portion of the VT—AUX line labeled as C. Upon a command to restart the internal combustion engine, the electrically-driven auxiliary hydraulic pressure pump is again boosted until a calibratable engine speed (‘RPM’) is reached, as shown at the portion of the VT—AUX line labeled as D. Once stable engine operation is reached, or after a calibrated delay, voltage supplied to the transmission auxiliary pump or VT—AUX, is 0.0 V, as shown at the portion of the VT—AUX line labeled as E.
In one vehicle application, once in the engine is off there can be delay timers implemented which delay a command to restart the engine after occurrence of an event. These delays can include the following events. When the operator shifts the gear selector (‘PRNDIL’ for Park-Reverse-Neutral-Drive-Intermediate-Low) to Park or Neutral, the internal combustion engine is commanded ON after a delay time in the range of four seconds. When the operator shifts to Reverse gear, the internal combustion engine is commanded ON after a transitional delay time of about a half-second (0.5 seconds). When the operator shifts to Intermediate or Low gear, the internal combustion engine is commanded ON after a delay time of about one second. When the PRNDIL is shifted back to Drive while the internal combustion engine is in an OFF state, the internal combustion engine is commanded ON after a delay time of about 0.05 seconds. These timers are referred to as the PRNDIL-based Restart timers.
The electrically-driven auxiliary hydraulic pressure pump may not have a prime, i.e. there may not be sufficient hydraulic fluid in the pump and hydraulic circuit to permit proper operation, when the internal combustion engine is commanded on after having been turned off during operation. When the auxiliary pump is unprimed, hydraulic pressure is not generated in the transmission when the pump is subsequently operated. The auxiliary pump can lose its prime due to air entrainment when the vehicle is stopped for a long duration or stopped on a large incline, when there is incomplete servicing of the plumbing of the auxiliary pump, when there is a pin leak in the auxiliary pump plumbing, and for other causes.
In some vehicle applications, hydraulic pressure is monitored, and loss of pressure due to a loss of prime in the auxiliary pump can be interpreted by the hybrid control system as the gear selector, or PRNDIL, being shifted into Park/Neutral or Intermediate/Low, and then to an undesired state, or, more directly, the gear selector being in an undesired state. An undesired state is registered when hydraulic pressure signature is not representative of any normal operation. In the undesired state, a fault code can be set, or the internal combustion engine is commanded ON immediately. When the gear selector is interpreted as being shifted into Park/Neutral or Intermediate/Low, and then to an undesired state, the internal combustion engine can be commanded ON when the operator does not expect it.
In any event, it is desirable for the hybrid control system to re-prime the auxiliary pump system, to provide reliable, predictable, and consistent engine stop-start behavior to avoid customer dissatisfaction and reduce warranty costs.
Therefore, a method and system are offered which address the above-stated concerns.